The present invention relates to a thermal printing control circuit and, more particularly, to a heat control circuit of a thermal printing head.
A thermal printing head comprises a plurality of print elements constituted by resistors arrayed in a line in correspondence with dots to be printed. Each print element is heated by applying a voltage pulse thereto for a short period of time at the timing for printing a corresponding dot. The dot is printed on print paper by keeping the print element at a temperature higher than the heat-sensitive temperature of the print paper for a certain period of time. Then, the heat of the print element is naturally dissipated upon removal of the voltage pulses and the temperature of the print element is dropped below the heat-sensitive temperature. The above operation is repeated each time a dot is printed.
Recently, as the printing speed of a printer is considerably increased, several problems have been posed in heat control of the above-described printing head.
"Thermal Printhead Drive Circuit for High Speed Pinting", IBM Technical Disclosure Bulletin, vol. 24, No. 1B, June 1981, pp. 646-648 describes a countermeasure for solving the problem of insufficient temperature rise caused by a decrease in duty cycle of an applied pulse due to high printing speed.
In contrast to the above problem, in a recent high-speed thermal printer, when, for example, linear printing is performed, since heating of print elements is successively repeated, heat of the print head is accumulated and the printing thickness of the dot is increased. This gradually causes unclear printing, thus posing another problem.
No proper countermeasure for solving this problem has yet been proposed by any prior art.